1. Field of the Invention
The present invention relates to a method of encapsulation of a thin-film lithium-ion type battery and, more specifically, to a method of encapsulation of such a battery directly on the substrate supporting it.
2. Discussion of the Related Art
Lithium-ion batteries have the advantage of comprising a solid non-flammable electrolyte, which further has a good ion conductivity over a wide range of temperatures. Such batteries could advantageously be used in mobile electronic devices such as cell phones or laptop computers.
To form thin-film lithium-ion batteries, typically batteries having dimensions smaller than 2.5×2.5 cm2, the use of techniques of sputtering through a shadow mask is known. Such techniques comprise placing a shadow mask above a support or substrate and sputtering, through this mask, the different layers forming the battery (active layers).
Generally, many thin-film batteries are formed simultaneously on a same substrate. Then, the substrate is diced to provide individual or elementary batteries which are placed in packages or encapsulated. Such packages connect the various active layers of the elementary batteries to the connection pads formed at the periphery of these packages.
FIG. 1 very schematically illustrates a packaged or encapsulated elementary lithium-ion battery. A stack 12 of the different active and contact layers of a lithium-ion type battery is formed on a portion of a substrate 10. Stack 12 may be formed, for example, by a succession of physical vapor depositions (PVD).
Stack 12 comprises a first cathode collector layer 14, a second cathode layer 16, a third layer 18 forming the battery electrolyte, a fourth layer 20 forming the anode of the battery, and a fifth layer 22 forming an anode collector. In the shown example, cathode collector layer 14 extends over the entire surface of substrate 10 while the other layers of stack 12 extend over a smaller surface area. This enables taking a contact on cathode collector layer 14.
The assembly of substrate 10 and of stack 12 is attached to the surface of a plate 24 forming the lower portion of the package. In the shown example, bonding wires 26 and 28 are attached at one of their ends, respectively, to cathode collector layer 14 and to anode collector layer 22 and, at their other end, to plate 24. Metal connections (not shown) are conventionally provided on and/or through plate 24 to transfer contacts 26 and 28 to the outside of the package. An insulating passivation and encapsulation material 30 is formed at the surface and on the sides of the assembly of substrate 10 and of stack 12 to complete the structure.
It should be noted that the package disclosed in relation with FIG. 1 is an example only of the many packages available to encapsulate lithium-ion type batteries. Especially, the connection by wires 26/28 between the active battery layers and support 24 may be performed by solder bumps. Further, insulating material 30 may be formed of a stack of several insulating passivation layers (for example, resins) and encapsulation layers (for example, ceramics).
In conventional battery packages such as that of FIG. 1, the surface area taken up by substrate 10 amounts to between half and three quarters of the total surface area of the encapsulated battery. For example, in a 3×3 mm2 package formed in LGA (Land Grid Array) technology, a substrate supporting a battery takes up approximately 66% of the package surface area and, in a 5×5 mm2 package forming in QFN (Quad Flat No leads) technology, the substrate takes up approximately 46% of the package surface area.
Further, the active portion of a lithium-ion type battery generally takes up three quarters of the surface area of the substrate on which it is formed. Thus, the active portion of an elementary battery takes up a surface area smaller than one quarter of the surface area of the final package. If the size of the elementary battery is decreased, the ratio between the active surface area of the battery and the package surface area further decreases, which makes it less useful to decrease the size of elementary batteries since the size of the encapsulated batteries decreases little. Thus, currently, a battery of lithium-ion type formed in a 3×3 mm2 package cannot store a capacity greater than one microampere-hour.
Before the packaging, problems appear as the substrate is being diced to form individual batteries. Indeed, the substrate dicing is generally followed by one or several steps of cleaning with aqueous compounds. During the cleaning operation(s), the insulating passivation and/or encapsulation material deposited on the active battery layers tends to dislocate or to separate from these active layers, which then find themselves in contact with the cleaning agents. Part of the active layers of a lithium-ion type battery being formed of highly-reactive lithium derivatives, it is as much as possible avoided to put these layers in contact with aqueous compounds.
There thus is a need for a method of encapsulation of a lithium-ion type battery which increases the ratio between the active surface area of the elementary batteries and the total surface area of the packaged battery and avoids the above-mentioned problems of separation.